DAHANEH, Afghanistan — Helicopter-borne U.S. Marines backed by Harrier jets stormed a Taliban-held town in southern Afghanistan before dawn today, the launch of a new operation to uproot Taliban fighters from a longtime base and provide security for next week's presidential election.

The troops exchanged heavy fire with insurgents, killing at least seven in an offensive they hoped would also cut Taliban supply lines and isolate their fighters.

Associated Press journalists traveling with the first wave said militants fired small arms, mortars and rocket-propelled grenades after helicopters dropped the troops over Taliban lines. Fighting lasted more than eight hours, as Harrier jets streaked overhead and dropped flares in a show of force.

The Taliban put up such fierce resistance that Marines said they suspected the militants knew the assault was coming.

Other Marines met heavy resistance as they fought to seize control of the mountains surrounding Dahaneh in the southern province of Helmand. Another convoy of Marines rolled into the town despite roadside bomb attacks and gunfire.

It was the first time NATO troops had entered Dahaneh, which has been under Taliban control for years.

U.S., NATO and Afghan troops are working to protect voting sites around the country so Afghans can take part in the country's second-ever direct presidential election on Aug. 20. Taliban militants have vowed to disrupt the elections, and attacks are on the rise.